As disclosed in U.S. Pat. No. 3,137,017, cross traffic legs are pivoted to the forward edge of the ramp and are normally biased forwardly to a supporting position. The cross traffic legs are coupled to the lip, so that as the ramp is elevated and the lip is extended, the cross traffic legs will automatically be pivoted to a retracted non-supporting position. After loading, as the truck pulls away from the dock and the lip pivots downwardly from the extended to the pendant position, the cross traffic legs are urged forwardly and engage the fixed supports on the frame of the dockboard to prevent further descent of the ramp.
The cross traffic legs, as illustrated in U.S. Pat. No. 3,137,017, are formed with series of abutments along the length of the legs so that the legs will not only support the ramp in a generally horizontal cross traffic position, but will also support the ramp in a series of below dock level positions.
It has been found that in situations where the truck pulls away from the loading dock while a load, such as a fork lift truck, is on the ramp, the cross traffic legs cannot respond quickly enough to catch or restrain the sudden descent of the ramp. The stored up energy in the compressed pneumatic tires of the lift truck tend to push the ramp down very suddenly when the truck pulls away from the loading dock and the lip and ramp are unsupported. Under these conditions of sudden drop, the lip tends to remain in its extended position with respect to the ramp, thereby maintaining the cross traffic legs in the retracted position and preventing the legs from engaging the supporting stops.
U.S. Pat. No. 3,368,229 is directed to a safety mechanism which is intended to prevent the sudden gravitational descent of the ramp caused by a load on the ramp when the truck pulls away from the dock. In accordance with the disclosure of U.S. Pat. No. 3,368,229, a tube is supported on the dock, and a strut moves up and down with the ramp and is slidable within the tube. The strut has downwardly facing ratchet teeth which coact with the an escapement device on the tube to support the ramp against descent and thereby prevent accidental descent of the ramp. An inertia responsive control for the escapement device keeps the pawl from engaging a ratchet tooth during gradual descent of the ramp, but on an initial sudden downward movement of the strut, the pawl engages the ratchet teeth and prevents the free downward movement of the ramp.
U.S. Pat. No. 3,877,102 is also directed to a safety construction for a dockboard which is associated with the cross traffic legs. In the disclosure of U.S. Pat. No. 3,877,102, the legs are coupled to the lip of the dockboard through a releasable latch mechanism which includes an acceleration responsive element. Under rapid descent of the ramp, the increased acceleration of the ramp will actuate the acceleration responsive element to release the latch mechanism and enable the legs to return under the force of biasing springs to the supporting position, independent of the position of the lip to, thereby support the ramp against further descent.